The present invention relates to a high efficiency semiconductor light emitting diode (LED) of the edge emitting type with one optically transparent confinement layer for use in high speed fiber optical communications.
The two types of semiconductor LED's most often used in fiber optical communication systems are the surface etched well emitter (Burrus type) and the edge-emitter. In the surface emitter, light is emitted from the surface of a planar LED structure along a path that is perpendicular to the p-n junction plane of the device. An optical fiber is aligned with this path to receive the emitted light. A well is etched into the top of the structure to enable the fiber to be as close as possible to the light emitting active region of the device.
The structure of the edge-emitter is very similar to that of a double heterojunction semiconductor laser. In the edge-emitter, the device is composed of layers of semiconductor material doped so that a p-n junction plane is formed. When the junction is forward biased, injected carriers (electrons and holes) recombine in the active layer and light is generated. The light is emitted from an edge of the device along a path which is parallel to the junction plane of the active layer. An optical fiber is aligned with this path at the edge of the device where the light is emitted.
Edge-emitters are potentially cheaper to fabricate and easier to package than surface emitters; however, the total optical power output of an edge-emitter is typically a fraction of that from comparable surface emitters. This lower output power level is caused by the reabsorption of light within the active region of the device. In one type of conventional edge emitting LED, the active layer is surrounded by two carrier confining layers. See Semiconductor Devices for Optical Communication, ed. H. Kressel, p. 38. These confining layers are relatively thick (on the order of 1.50 .mu.m), thereby confining not only the excess injected carriers, but also spontaneously emitted photons generated during recombination. Photons cannot escape from the active layer in this structure, because they are totally internally reflected at the dielectric interface between the active layer and the confining layers. The photons are therefore trapped in the active layer before they eventually leave the device at the exit facet. Because the material composing the active layer is energy absorbing, device efficiency is relatively low.
An improved structure was proposed by Y. Horikoshi, et al. in Japanese Journal of Applied Physics, 15, 485 (1976) in which a guide layer was incorporated. This device provides a low absorption path by which photons can exit the LED chip, but carrier confinement provided by the optical guide layer is not adequate. As a result device internal quantum efficiency is lowered and device temperature sensitivity (i.e., drop in output power with increasing junction temperature) is increased.